This may qualify as a ridiculous question, but I'm looking to lower the RPM necessary to build boost on my STS GT67. I'm wrapping the exhaust this weekend. Is an A/R change another way to accomplish this? I don't know what the current ratio is on the turbo, but it's the GT67 that came straight from STS as an option when I bought the kit.
If I were to change the A/R, what ratio would I want to go with, and where would I buy the adapter?


Ross

 

A/R is dictated by the exhaust housing, a lower A/R (.68) would spool faster but run out on the top end faster whereas a higher (.96) A/R will spool Slow

 

I think my STS GT67 came with a .81 exhaust housing. I've wrapped half the exhaust so far, but I am looking at stepping down one size. Would any turbo dealer be able to get one?

 

Yes, any shop that sells turbos can get the turbine housing in whatever A/R's are available. For the GT67, you want a T04E housing (most when sized for a 6.0L should be .96 A/R -- I think STS sends the .81 for 5.3s)

As far as A/R, my GT67 on the LQ9 spooled to full boost at 3000 rpm with the .96 A/R housing. If your spool is slower than that with a 6.0 you need to check for exhaust leaks before going any further.

Wrapping the exhaust keeps heat in (duh ) and that has the same effect as reducing the A/R. You can keep shrinking the A/R and get spool closer and closer to idle, but at the cost of peak power. Realistically, I unless you're going to run 14-15psi, I'd think you can go to whatever A/R you want with a GT67 on 6.0L. Not true of 5.3 of course.

I'd try a .81 housing first. Turbine housings should be about a buck and a half to two bills, so you can swamp around 2-3 times for cheap. I plan on keeping a set of "street" turbine housings and a set for race day.

 

Erik- Thanks so much for the info! I'll double check the A/R on the GT67 with a phone call to STS tomorrow. If it's the .96 I have, I'll give the .81 a go... Looks like there are several companies online that sell turbine housings.
Good question about full boost by 3k . I'll have to check that tomorrow. Problem is, I have an analog boost controller that allows boost to be adjusted by gain, etc. Makes it more difficult to know whether you're actually getting the boost dialed in on the controller. I CAN say, that with boost set to 10psi, I'm only building ~4psi or so in first gear before the change to second.
Just in case, what method do you recommend for an exhaust leak check?

 

It's not super fun hehe

Idle the truck in park and crawl under it feel around the welds (don't touch, but "hover" your hand about 1.5" from the pipe) all the way around. A big leak will feel like someone blowing on your hand, a small leak will be like puffs.

4psi in first isn't an A/R problem. I'd check that boost controller. I had a similar problem way back when and it turned out to be a leaky connection to the wastegate (too big of a line and it wouldn't seal tight enough for higher boost).

 

That does sound fun!!
Checking it this afternoon...let ya know how it turns out. Thanks.

 

You can also get a buddy to stuff a rag in the tail pipe and try to hold it in there while it's running. This will create a lot more back pressure at idle and make a leak hiss pretty loud and easier to find.

 

Check the charge pipes too. I've managed to find a few leaks there in the past.